Fuel efficiency of motor vehicles can be improved by integrating a thermoelectric generator in heated areas of the vehicle. The thermoelectric generator converts waste heat from the internal combustion engine into electricity according to the principles of the Seebeck Effect. Since excess heat is present in numerous locations in the vehicle, in particular in the exhaust system, using a thermoelectric generator in the exhaust converts heat energy that would ordinarily be wasted into useful electric energy.
Conventional thermoelectric generators include a hot-side heat exchanger and a cold-side heat exchanger, which are separated by a plurality of semiconductor modules. In a typical thermoelectric generator, n-type semiconductors and p-type semiconductors are electrically connected in series such that the n-type and p-type semiconductors alternate along the electric path. The p-type and n-type semiconductors are typically arranged between the hot-side heat exchanger and the cold-side heat exchanger so as to be thermally in parallel with each other.
When heat passes through the thermoelectric generator, the charge carriers of the semiconductors within the generator diffuse from the hot-side heat exchanger to the cold-side exchanger. The build-up of charge carriers results in a net charge, producing an electrostatic potential, while the heat transfer drives a current through the series-connected semiconductor elements. This production of electric energy is known as the Seebeck Effect.
In a vehicle exhaust system, temperatures can reach 700° C. (˜1300° F.) or more, and the resulting temperature difference between exhaust gas on the hot side and coolant on the cold side is therefore several hundred degrees. A vehicle exhaust having a thermoelectric generator uses the Seebeck principle to convert the temperature difference in the exhaust system through the p-type and n-type semiconductors of the thermoelectric generator into a potential difference. The resulting electric energy can be used to power electrical components in the vehicle, charge the battery in the vehicle, or, in hybrid vehicles, power the vehicle drivetrain.
The efficiency of heat to electricity conversion for a given semiconductor material is quantified by a dimensionless parameter called the figure of merit (ZT), which is calculated according to the equation:ZT=δS2T/(κel+κlat),where σ is the electrical conductivity, S is the Seebeck coefficient, T is temperature, κel is the electronic thermal conductivity, and κlat is the lattice thermal conductivity.
A material having a greater figure of merit results in more energy produced at a given temperature differential between the hot and cold side heat exchangers. As such, it is desirable for the semiconductor elements of a thermoelectric generator to be formed of materials with high figure of merit in order to improve efficiency and electricity generation in a thermoelectric generator.